Public Health England is working with Brentwood School after a sickness outbreak last week.
The school said they believed the cause was a virus and absence levels among staff and pupils at the school were higher than normal on Friday.
A spokesman for the school said: “Nonetheless, the pupil absences represented only a small percentage of the total population and there was no need to bring in supply staff to cover lessons.
“The school was able to provide the full range of Friday lessons and activities. However, some of the staff and students who were absent reported symptoms of sickness, although at the time of writing it is too early to confirm the reason for every absence.
“The pattern of illness suggests that a virus is the most likely source and the school is working with Public Health England and the local Environmental Health Department to reduce the likelihood of further infection amongst staff and pupils.”
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